Epoxy/GO nanocomposite adhesives reinforced with waste blends of PET/GTR: Evaluation of mechanical properties
Introduction
Epoxy resins can be obtained from natural [[1], [2], [3]] or synthetic [4,5] sources. It is well-known nowadays that a material with higher properties is the result of incorporation of nanoparticles into epoxy [[6], [7], [8]]. Epoxy has been welcomed as the most important thermosetting resin used in development of high-performance adhesives [[9], [10], [11]]. Epoxy is always the first choice between resins to bond fragments of materials such as steel, copper, wood, iron, cement, plastics and composites with each other [12,13]. When expecting high modulus and tensile strength from epoxy-based resins, the prerequisite would be the formation of three-dimensional network applying an appropriate curing agent together with an additives to the system [14,15]. When using epoxy, however, it can be experienced that it is relative fragile and undertakes crack initiation and crack growth [[16], [17], [18]]. Simultaneous use of epoxy and other types of resin, appropriate curing agent, cure modifiers, and organic or mineral fillers were underlined as the main route to tackle such problem [15,[19], [20], [21]]. It some works the combination of a low-cost secondary micro-particles dispersible in a polymer with inherently higher Young’s modulus and/or degradation temperature greatly were applied to enhance the toughness of epoxy [17,[22], [23], [24]]. Nevertheless, using micro- of nanoparticles to epoxy should be performed at an optimal loading level to avoid aggregation and/or agglomeration of particles that deteriorate mechanical properties of epoxy composites [[25], [26], [27], [28]].
Graphite and graphene oxide (GO) from carbon family have entered the polymer composite manufacturing game after carbon nanotubes, and due to their outstanding properties made development of high-performance polymer nanocomposites flourishing [[29], [30], [31]]. Adding small amount of GO would improve significantly the electrical and mechanical properties. The hydroxyl, carboxylic and/or epoxide groups are mainly located on the surface or edges of the GO platelets making GO reactive toward curing moieties in epoxy adhesive formulations [32]. Such functional groups can improve interactions between the GO nanosheets and polar groups of polymer chains [[33], [34], [35]]. The lamellar GO platelets with high toughness in comparison with polymer assist in bonding GO with the resin making facile the stress transfer across the epoxy/GO interface [36].
The mechanical properties and toughness of rubber-modified epoxy/GO adhesives revealed that Young’s modulus of epoxy was increased even by adding 0.1 wt.% of GO from 2.46 to 2.56 GPa, but with no obvious rise in tensile strength value [36]. Pyrolysis of epoxy composites containing waste of poly(ethylene terephthalate) (W-PET) was indicative of thermal stability improvement by increase of W-PET content. It was also reported that increasing the amount of W-PET was responsible for an improved tensile strength at the whole testing temperature interval [37]. To boost dispersion of a polymer in epoxy resin and thereby the enhancement of the mechanical properties of the resulting nanocomposite some suggested reinforcing the resulting epoxy/polymer with the GO nanosheets [38]. The effective action of GO with poly(vinylimidazole), which resulted in good dispersion of GO nanoparticles in the epoxy matrix, improved the curing by lowering the activation energy. The results of the tensile measurements showed that incorporation of 0.25 wt.% of GO into the epoxy matrix increased the tensile strength by 60% and the modulus by 45.5% compared to pure epoxy. The effect of GO surface energy on the loss in mechanical energy of the epoxy was also addressed by the researchers, where a direct relation between GO content and improvement in the impact strength was observed (adding 1 wt.% of GO to epoxy increases the strength by 80% that is from 1.04 to 7.27 MPa.) [39]. In another research, thermal stability and thermal conductivity of GO-incorporated epoxy were assessed and the properties were significantly improved at very low loading [40]. To enhance the properties of polymer/GO composites, it is essential to have GO in the form of intercalated single platelets well-dispersed in the polymer matrix, as GO monolayer has a strong mechanical strength [41]. The effect of GO on the fracture toughness, mechanical and thermal properties of epoxy toughened with carboxyl-terminated acrylonitrile butadiene (CTBN) was also reported [42]. Among two different GO platelets with diameters of less than 1 and 5 μm separately added to the epoxy/CTBN the former significantly improved the fracture toughness through crack deviation, layer failure, and layer separation mechanisms [40,43,44]. Scanning electron microscopy (SEM) confirmed interfacial interaction enhancement resulting from combined use of GO and rubber particles. Based on literature survey, it should be recognized that the use of waste materials together GO would lead to development of epoxy-based adhesives with superior mechanical properties [[45], [46], [47]]. In a previous report it was revealed that GO is a good candidate for reinforcing epoxy adhesives containing recycled/waste polymers [41].In this work, the effect of GO and the blend of W-PET/GRT in different ratios on the mechanical properties of epoxy resin was studied in the absence and presence of GO platelets to optimize tensile (on dumbbell samples) and single-lap shear strength (on steel-epoxy/carbon fiber composite samples) properties.
Section snippets
Materials
Epoxy resin (DGEBA) of EPON828 grade, manufactured by DOW, was applied as a glue base. Hireontel phenolic resin was applied to improve thermal properties of the adhesive. Triethylenetetramine (TETA) was used as a dough curing agent. Waste tire powder produced by Isatis Yazd was employed as a toughening agent, and waste of poly(ethylene terephthalate) (W-PET) from Zamzam Company was applied as the second toughening agent. Graphene oxide (GO) from Graphex was used as nanoparticle. Other solvents
Tensile and lap-shear properties of adhesive series of B1
Considering the fact that samples of series B1 blends were mainly composed of W-PET (80 wt.% W-PET and 20 wt.% GTR + SEBS-g-MAH), properties were expected to be governed by the content of W-PET. Tensile strength measurement was done to assess mechanical behavior of blends of series B1. According to Fig. 1, Sample 2 has the highest strain at break, toughness, and tensile strength values among series of B1 blend samples. Increase in the strength due to increasing W- W-PET content is indicative of
Conclusion
The ground rubber tire (GRT) and polyethylene terephthalate bottle wastes (W-PET) are used as toughening agents in the formulation of epoxy-phenolic adhesives. To have an improved interaction between waste rubber phase and epoxy matrix, the waste GRT powder and W-PET elastomer were compatibilized with SEBS-g-MAH in a twin-screw extruder before being applied in adhesive formulations. GO nanosheets were incorporated into the system once the best epoxy adhesive with the highest tensile and
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